Production method for compound semiconductor single crystal

ABSTRACT

A method for producing a compound semiconductor single crystal by a liquid encapsulated Czochralski method, including containing a semiconductor raw material and an encapsulating material in a raw material melt-containing portion having a first crucible having a bottom and a cylindrical shape and a second crucible disposed within the first crucible and having a communication hole communicating with the first crucible in a bottom portion thereof; melting the raw material by heating the raw material melt-containing portion; and growing a crystal by making a seed crystal contact with a surface of the raw material melt in a state covered with the encapsulating material and by pulling up the seed crystal. A heater temperature is controlled so that a diameter of a growing crystal becomes approximately equal to an inner diameter of the second crucible, and the crystal is grown by maintaining a surface of the growing crystal in a state covered with the encapsulating material until termination of crystal growth.

FIELD OF THE INVENTION

The present invention relates to a method for producing a compoundsemiconductor single crystal, and more particularly, to a techniqueusefully applied to a method for producing, for example, a ZnTe systemcompound semiconductor single crystal by the liquid encapsulatedCzochralski (LEC) method.

BACKGROUND ART

Presently, a ZnTe system compound semiconductor single crystal is deemedas a promising crystal which can be applied to a pure greenlight-emitting element.

In general, the ZnTe system compound semiconductor single crystal isproduced by a vapor phase epitaxy method in which a ZnTe polycrystal asa raw material disposed at one end in a quartz ampoule is heated to besublimed at a temperature around a melting point thereof, and a ZnTesingle crystal is deposited on a substrate arranged on an opposite sideof the ampoule. This method makes it possible to obtain a rectangularZnTe single crystal substrate of approximately 20 mm×20 mm at themaximum. Recently, to further enhance the light-emitting characteristicof the crystal as a light-emitting element, it has been devised toincrease the electrical conductivity of a crystal, and a method ofadding an impurity, such as phosphorus, arsenic or the like, to thecrystal is carried out, as the method for increasing the electricalconductivity.

Further, it is also possible to make use of the vertical Bridgeman (VB)method or the vertical gradient freezing (VGF) method to grow a ZnTesystem compound semiconductor single crystal. In the VB method and theVGF method, since an impurity can be added during crystal growth, thesemethods are advantageous in that it is easy to control the electricalconductivity of a crystal through addition of an impurity to thecrystal. Further, it is also devised to cover the liquid surface of araw material melt with an encapsulating material so as to prevent animpurity from being mixed into the melt from above to inhibit forming ofa single crystal, and to suppress temperature fluctuation in the melt.

However, in the case of growing a ZnTe system compound semiconductorsingle crystal by the vapor phase epitaxy method, it is difficult to adda desired impurity during crystal growth, and hence difficult to controlthe resistivity of the ZnTe system compound semiconductor singlecrystal. Further, in the vapor phase epitaxy method, the growth rate ofa ZnTe crystal is extremely slow, and hence it is difficult to obtain asingle crystal having a sufficient size, which results in lowproductivity.

Moreover, even if a relatively large substrate of approximately 20 mm×20mm can be obtained by growing the ZnTe system compound semiconductorsingle crystal, the low productivity makes the substrate very expensive,which offers a barrier to development of devices using the ZnTe systemcompound semiconductor single crystal.

For the above reasons, production of the ZnTe system compoundsemiconductor single crystal by the vapor phase epitaxy method is notpractical as a method for industrial production.

On the other hand, in production of the ZnTe system compoundsemiconductor single crystal by the VB method or the VGF method, it ispossible to grow a large-sized crystal, but since the crystal is grownby being cooled in the state covered by the encapsulating material, adifference in the coefficient of thermal expansion between theencapsulating material and the growing crystal often causes cracking inthe crystal.

Further, in the LEC method, similarly to the VB method and the VGFmethod, it is also possible to add an impurity, and hence the LEC methodis advantageous in that it is easy to control the electricalconductivity of a crystal through addition of an impurity to thecrystal. However, almost no example has been known so far in which alarge-sized ZnTe compound semiconductor single crystal is grown by theLEC method.

An object of the invention is to provide a method for producing acompound semiconductor single crystal, which is capable of growing alarge-sized ZnTe system compound semiconductor single crystal or otherkinds of compound semiconductor single crystals while maintainingexcellent crystal quality.

DISCLOSURE OF THE INVENTION

In order to attain the above object, the present invention provides amethod for producing a compound semiconductor single crystal by a liquidencapsulated Czochralski method, comprising: containing a semiconductorraw material and an encapsulating material in a raw materialmelt-containing portion comprising a first crucible and a secondcrucible, the first crucible having a bottom and a cylindrical shape,and the second crucible being disposed in an inside of the firstcrucible and having a bottom portion thereof provided with acommunication hole communicating with the first crucible; melting theraw material by heating the raw material melt-containing portion; andgrowing a crystal by making a seed crystal be in contact with a surfaceof the raw material melt in a state covered with the encapsulatingmaterial and by pulling up the seed crystal, wherein a heatertemperature is controlled so that a diameter of a growing crystalbecomes approximately equal to an inner diameter of the second crucible,and the crystal is grown by maintaining a surface of the growing crystalin a state covered with the encapsulating material until termination ofcrystal growth.

According to this method, since components can be prevented from beingevaporated from the crystal surface, a temperature gradient in theencapsulating material can be made extremely small, and a crystal havingexcellent quality can be grown. Further, it is possible to suppresstemperature fluctuation in the raw material melt by reducing thetemperature gradient in the encapsulating material, so that even with amaterial, such as ZnTe or the like, which is conventionally consideredto be difficult to obtain a single crystal, it is possible to grow asingle crystal from a seed crystal thereof.

Furthermore, the diameter of the growing crystal can be madeapproximately equal to the inner diameter of the second crucible bycontrolling the heater temperature. Therefore, it is possible to easilyobtain a single crystal having a desired diameter, and moreover, it ispossible to basically dispense with a relatively complicated temperaturecontrol program or the like for controlling the diameter of the growingcrystal.

Further, an amount of the encapsulating material to be added may be setto an amount such that the encapsulating material is capable of fillinga space generated between the growing crystal and the second crucible inaccordance with the crystal growth and covering an entire surface of thegrowing crystal. In other words, the amount of the encapsulatingmaterial to be added is adjusted such that even after the encapsulatingmaterial fills the space generated between the growing crystal and thesecond crucible, the encapsulating material remains on the upper surfaceof the growing crystal. Thereby, since the growing crystal is held in astate positively covered with the encapsulating material, componentelements of the growing crystal will not be evaporated.

Further, a crucible having a tapered structure in which an innerdiameter of a bottom portion of the crucible is smaller than an innerdiameter of a top portion of the crucible may be used as the secondcrucible. This makes the diameter of the growing crystal being pulledupward smaller than the inner diameter of a portion of the secondcrucible at a corresponding position. Therefore, since the growingcrystal is prevented from being brought into contact with the cruciblewall surface, except at a growth interface, it becomes possible toobtain a crystal excellent in quality.

Further, it is preferred that the second crucible has a side facethereof tilted with respect to a vertical direction within a range of0.2° to 10°. This makes the volume of the space generated between thegrowing crystal and the second crucible relatively small, so that it ispossible to cover the entire surface of the growing crystal with theencapsulating material without using an extremely large amount of theencapsulating material.

Further, the crystal growth may be performed in a state of the secondcrucible being dipped in the raw material melt contained in the firstcrucible to a depth within a range of 10 mm to 40 mm, and a diameter ofthe communication hole may be not more than ⅕ of the inner diameter ofthe second crucible. Thereby, since it is possible to efficientlysuppress temperature fluctuation in the raw material melt in the secondcrucible, a single crystal having excellent quality can be grown. Inaddition, since the communication path communicating with the firstcrucible is limited, even when a contaminant or the like has mixed intothe raw material melt in the second crucible, it is possible to removethe contaminant from the second crucible into the first crucible bypulling up the second crucible, to thereby prevent the contaminant frombeing mixed into the crystal being grown.

Furthermore, although a difference in the concentration of the impurityin the raw material melt is caused between the first crucible and thesecond crucible when an impurity as a dopant is added to the rawmaterial melt, it becomes possible to control the difference in theconcentration of the impurity in the melt and keep constant theconcentration of the impurity in the raw material melt in the secondcrucible by modifying the size of the communication hole of the secondcrucible under the condition of the size not more than ⅕ of the innerdiameter of the second crucible.

Further, a temperature gradient in the raw material melt may be set toat least not more than 20° C./cm, whereby it is possible to prevent apolycrystal or a twin crystal from occurring. It should be noted thatsince the growing crystal is always covered with the encapsulatingmaterial, even if the temperature gradient is reduced, there is no fearof decomposition of the growing crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing the arrangement of a crystalgrowth apparatus used in an embodiment of the present invention; and

FIG. 2 is an enlarged view showing a raw material melt-containingportion of the crystal growth apparatus shown in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of the present invention will beexplained with reference to the drawings.

FIG. 1 is a view schematically showing the arrangement of an embodimentof a crystal growth apparatus according to the present invention, andFIG. 2 is an enlarged view showing a raw material melt-containingportion.

The crystal growth apparatus 100 comprises a high pressure container 1,a thermal insulation member 2 and a heater 3 arranged within the highpressure container 1 concentrically with the same, a rotating shaft 4arranged vertically in a central portion of the high pressure container1, a susceptor 13 arranged on the upper end of the rotating shaft 4, apBN outer crucible (first crucible) 5 having a bottom and a cylindricalshape and fitted in the susceptor, a pBN inner crucible (secondcrucible) 6 arranged within the outer crucible 5, and a rotating liftshaft 7 vertically arranged above the inner crucible 6 and having a seedcrystal holder 8 attached to the lower end thereof, for holding a seedcrystal 9.

The inner crucible 6 has a bottom surface thereof formed with acommunication hole 6 a which communicates with the outer crucible 5, anda raw material melt 12 can move from the outer crucible 5 to the innercrucible 6 via the communication hole 6 a. It should be noted that theinner crucible 6 is fixed to the outer crucible 5 or a jig other thanthe outer crucible 5 by a suitable holder (not shown).

Further, the inner crucible 6 has a tapered structure in which an innerdiameter of a bottom portion thereof is smaller than an inner diameterof a top portion thereof, and hence the diameter of a growing crystalbeing pulled upward becomes smaller than that of the second crucible ata corresponding position, which prevents the growing crystal from beingbrought into contact with the wall surface of the second crucible,except at a growth interface of the growing crystal. Further, it ispreferred that the inner crucible has a side face thereof tilted withrespect to the vertical direction within an range of 0.2° to 10° to forma taper, so as to make the volume of a space generated between thegrowing crystal and the second crucible during crystal growth relativelysmall to thereby reduce the amount of the encapsulating materialentering the space.

The rotating lift shaft 7 is connected to a driving portion (not shown),arranged outside the high pressure container, to form a rotating liftmechanism. The rotating shaft 4 is connected to a driving portion (notshown), arranged outside the high pressure container 1, to form acrucible-rotating mechanism as well as a susceptor-lifting mechanism. Itshould be noted that the rotational motion and the lifting motion of therotating lift shaft 7 and the crucible-rotating shaft 4 are set andcontrolled independently of each other.

By using the crystal growth apparatus described above, it is possible topull a single crystal rod grown from the seed crystal while rotating thesame, and thereby grow a highly pure single crystal at the lower endthereof by the liquid encapsulated Czochralski method.

Next, a method for producing a ZnTe compound semiconductor singlecrystal, as an example of a compound semiconductor, by using the crystalgrowth apparatus 100 will be described in detail.

In the embodiment, the outer crucible 5 is implemented by a pBN cruciblehaving a size of 100 mmφ in inner diameter×100 mm in height×1 mm inthickness, while the inner crucible 6 is implemented by a pBN cruciblehaving a tapered structure and a size of 54 mmφ (bottom portion d2) to56 mmφ (top portion d3) in inner diameter×100 mm in height×1 mm inthickness. In this case, the tilt angle θ of the side face of the innercrucible 6 is approximately 0.57° with respect to the verticaldirection.

The communication hole 6 a having a diameter (d1) of 10 mm is formed inthe central portion of the bottom surface of the inner crucible 6. Itshould be noted that the size of the communication hole 6 a is notlimited to 10 mm, but the communication hole 6 a may be formed to haveany size so long as the size not more than ⅕ of the inner diameter ofthe inner crucible 6.

First, Zn having a purity of 6N and Te having a purity of 6N as rawmaterials were placed in the outer crucible 5 and the inner crucible 6in a total amount of 1.5 kg such that they were equimolar, and the rawmaterials were covered with 400 g of an encapsulating material (B₂O₃) 11to form an encapsulating material layer having a thickness of 35 mm. Itshould be noted that after melting the raw materials by the heater 3,the inner crucible 6 was fixed by a holder such that it was in a statedipped in the raw material melt to a depth of 20 mm from the liquidsurface of the raw material melt. Further, while the raw material meltgradually decreased with crystal growth, the dipped state of the innercrucible 6 was controlled by lifting the rotating shaft 4 to move upwardthe susceptor 13 (outer crucible 5). For instance, the inner crucible 6was held in a state dipped in the raw material melt to a depth within arange of 10 to 40 mm from the liquid surface of the raw material melt.

Next, the crucibles 5 and 6 were disposed on the susceptor 13, and thehigh pressure container 1 was filled with an inert gas (for example, Ar)so that the pressure within the high pressure container 1 would beadjusted to a predetermined pressure. Then, the crucibles 5 and 6 wereheated by the heater 3 at a predetermined temperature, with the surfacesof the raw materials held down by the encapsulating material, so that Znand Te were melted and synthesized directly.

Then, after the raw material melt has been held for a predetermined timein the melted state, the seed crystal 9 was brought into contact withthe surface of the raw material melt. Here, a seed crystal having a(100) crystal orientation was used as the seed crystal. Further, theseed crystal 9 was covered by a cover made from molybdenum (not shown),for prevention of decomposition thereof.

Thereafter, the rotating lift shaft 7 was rotated at a rotational speedof 1 to 2 rpm and pulled up at a rate of 2.5 mm/h to form a shoulderportion of the crystal. Subsequently, after the shoulder portion wasformed, the crucible-rotating shaft was rotated at a rotational speed of1 to 5 rpm and pulled up at a rate of 2.5 mm/h to form a body portion ofthe crystal. In this case, the diameter of the body portion of thegrowing crystal 10 was approximately the same as the inner diameter ofthe inner crucible 6 as shown in FIG. 2. Therefore, it was possible toobtain a crystal having a desired diameter easily without executingcomplicated diameter control by the pulling rate and the rotationalspeeds of the crucibles and the rotating lift shaft.

Further, since the inner crucible 6 is formed to have the taperedstructure, it was possible to prevent the growing crystal 10 from beingbrought into contact with the crucible wall surface at any other portionthan the growth interface when the growing crystal 10 was pulled upwardin accordance with crystal growth. More specifically, as shown in FIG.2, a gap was formed between the growing crystal 10 and the innercrucible 6, and the encapsulating material entered the gap to cover thesurface of the growing crystal. Thereby, it was possible to prevent thegrowing crystal from being brought into contact with the wall surface ofthe inner crucible 6 to cause deterioration of crystal quality.

Moreover, since the gap between the growing crystal 10 and the innercrucible 6 was small, the amount of encapsulating material 11 in the topportion of the crystal moved into the gap was small, and hence thecrystal surface was always held in the state covered with theencapsulating material 11. Thereby, since component elements of thegrowing crystal 10 was prevented from being evaporated, and atemperature gradient in the encapsulating material could be madeextremely small a grown crystal excellent in quality could be obtained.

Although in the present example, the tilt angle θ of the side face ofthe inner crucible 6 with respect to the vertical direction is set to0.57°, the tilt angle may be within a range of 0.2° to 10°.

Further, from the fact that the temperature fluctuation in the rawmaterial melt in the inner crucible 6 was approximately 0.5° C., andthat the temperature fluctuation in the raw material melt between theinner crucible 6 and the outer crucible 5 was approximately 1 to 2° C.,it could be confirmed that the double crucible structure made itpossible to suppress temperature fluctuation in the inner crucible 6.

Furthermore, the temperature gradient in the raw material melt duringcrystal growth was not more than 10° C./cm, and since the surface of thegrowing crystal 10 was always covered with the encapsulating material11, no decomposition of the crystal occurred.

The crystal growth was performed by the liquid encapsulated Czochralskimethod as described above, and after the crystal having been grown, thegrown crystal 10 was separated from the encapsulating material 11,whereby ZnTe compound semiconductor crystal without any crack wasobtained. The obtained ZnTe compound semiconductor crystal was anextremely excellent single crystal without any polycrystal or twincrystal. Further, the grown crystal had a size of 54 mmφ in diameter×60mm in length, which means that an increase in the size of the ZnTesystem compound semiconductor crystal, conventionally considered to bedifficult to attain, could be realized.

In the above, the present invention is explained in detail on the basisof the example made by the present inventors. However, the presentinvention is not limited to the above-described example.

For instance, in the above-described embodiment, the inner crucible 6has the bottom surface thereof formed with the single communication holehaving a diameter of 10 mmφ. However, the number of communication holeis not limited to one, but even if a plurality of communication holesare provided, it is considered possible to obtain effects, such assuppressing temperature fluctuation, and the like.

Further, by adding an impurity as a dopant in the raw material melt, itis possible to easily control the electrical conductivity of thecrystal. In this case, although a difference in the concentration of theimpurity in the raw material melt is caused between in the outercrucible 5 and in the inner crucible 6, it becomes possible to controlthe difference in the concentration of the impurity in the raw materialmelt between the two crucibles and keep constant the concentration ofthe impurity in the raw material melt in the inner crucible 6 bymodifying the size of the communication hole formed in the secondcrucible within a range of not more than ⅕ of the inner diameter of thesecond crucible.

According to the present invention, the method for producing a compoundsemiconductor single crystal by a liquid encapsulated Czochralskimethod, comprises: containing a semiconductor raw material and anencapsulating material in a raw material melt-containing portioncomprising a first crucible and a second crucible, the first cruciblehaving a bottom and a cylindrical shape, and the second crucible beingdisposed in an inside of the first crucible and having a bottom portionthereof provided with a communication hole communicating with the firstcrucible; melting the raw material by heating the raw materialmelt-containing portion; and growing a crystal by making a seed crystalbe in contact with a surface of the raw material melt in a state coveredwith the encapsulating material and by pulling up the seed crystal. Aheater temperature is controlled so that a diameter of a growing crystalbecomes approximately equal to an inner diameter of the second crucible,and the crystal is grown by maintaining a surface of the growing crystalin a state covered with the encapsulating material until termination ofcrystal growth. This makes it possible to easily obtain a crystal havinga desired diameter without executing complicated diameter control, andprevent evaporation of component elements from the crystal surfaceduring crystal growth, thereby growing a crystal excellent in quality.

Moreover, the double-crucible structure makes it possible to suppresstemperature fluctuation in the raw material melt received in thecrucibles, so that production of a twin crystal and a polycrystal can beprevented, so that an extremely excellent crystal can be obtained.

INDUSTRIAL APPLICABILITY

The present invention is by no means limited to the producing of a ZnTecompound semiconductor single crystal, but it can be applied toproducing of not less than ternary ZnTe system compound semiconductorsingle crystals containing ZnTe or other compound semiconductor singlecrystals.

1. A method for producing a compound semiconductor single crystal by aliquid encapsulated Czochralski method, comprising: containing asemiconductor raw material and an encapsulating material in a rawmaterial melt-containing portion comprising a first crucible and asecond crucible, the first crucible having a bottom and a cylindricalshape, and the second crucible being disposed in an inside of the firstcrucible and having a bottom portion thereof provided with acommunication hole communicating with the first crucible; melting theraw material by heating the raw material melt-containing portion; andgrowing a crystal by making a seed crystal be in contact with a surfaceof the raw material melt in a state covered with the encapsulatingmaterial and by pulling up the seed crystal, wherein a crucible having atapered structure in which an inner diameter of a bottom portion of thecrucible is smaller than an inner diameter of a top portion of thecrucible and in which a side face thereof tilted with respect to avertical direction within a range of 0.2° to 10° C., and a diameter of acommunication hole may be not more than ⅕ of the inner diameter of thecrucible may be used as the second crucible, and when the body of acrystal is grown, a heater temperature is controlled so that on aninterface between a growing crystal and the raw material melt, thecrystallization is advanced until reaching an inner wall of the secondcrucible, and the crystallization is done in such a way that a diameterof the body of the growing crystal is consistent with an inner diameterof the second crucible on the interface and the diameter of the body ofthe growing crystal is confined by the inner wall of the secondcrucible, while the crystal is grown by maintaining a surface of thegrowing crystal in a state covered with the encapsulating material untiltermination of crystal growth.
 2. The method for producing a compoundsemiconductor single crystal as claimed in claim 1, wherein an amount ofthe encapsulating material to be added is set to an amount such that theencapsulating material is capable of filling a space generated betweenthe growing crystal and the second crucible in accordance with thecrystal growth and covering an entire surface of the growing crystal. 3.The method for producing a compound semiconductor single crystal asclaimed in claim 1, wherein the crystal growth is performed in a stateof the second crucible being dipped in the raw material melt containedin the first crucible to a depth within a range of 10 mm to 40 mm. 4.The method for producing a compound semiconductor single crystal asclaimed in claim 2, wherein the crystal growth is performed in a stateof the second crucible being dipped in the raw material melt containedin the first crucible to a depth within a range of 10 mm to 40 mm. 5.The method for producing a compound semiconductor single crystal asclaimed in claim 1, wherein a temperature gradient in the raw materialmelt is set to at least not more than 20° C./cm.
 6. The method forproducing a compound semiconductor single crystal as claimed in claim 2,wherein a temperature gradient in the raw material melt is set to atleast not more than 20° C./cm.
 7. The method for producing a compoundsemiconductor single crystal as claimed in claim 3, wherein atemperature gradient in the raw material melt is set to at least notmore than 20° C./cm.
 8. The method for producing a compoundsemiconductor single crystal as claimed in claim 4, wherein atemperature gradient in the raw material melt is set to at least notmore than 20° C./cm.